The invention relates to x-ray systems, and specifically to a real-time x-ray imaging system having a superior scintillating screen for converting x-ray photons into visible light.
Generally, x-ray imaging is accomplished by passing x-rays emitted from a standard x-ray generated through a component or body part that is to be investigated. The amount of power that is required by the x-ray generator to generate sufficient x-ray photons to penetrate or sufficiently illuminate the subject depends on the thickness and/or density of the subject. The image is generally captured on x-ray film, which is exposed by visible light, which is emitted from a device known as a scintillating screen. In many instances it is desirable to observe the x-ray pattern in real-time, as in the case of a quality control program for industrial use, or for medical purposes, such as in the case of invasive surgery.
In order to generate a real-time image, the x-rays are directed through the subject to a scintillating screen, which is viewed by a video camera. Regardless of whether the image is captured on film or by a video camera, a basic limitation of known x-ray systems is the scintillating screen, which typically converts only about 35-40 percent of the x-ray photons striking the screen into visible light which may be captured by film or video equipment.
Known scintillating screens generally use a plasticized material that is coated with yet another material that phosphors, or glows, producing photons and visible light when impacted by x-ray photon radiation. The capacity of such screens to emit visible light depends on the amount of radiation that impacts the screen after passing through the subject. In the case where the subject is captured on film, the film generally has a narrow latitude, which requires that large or particularly dense subjects be exposed to higher levels of radiation, or for longer periods of time, or both, in order to produce a usable, visible-light radiation from the screen. In the case where industrial, non-electronic, objects are the subject, this is generally not a problem. However, in the case where animal tissue, and particularly human tissue is the subject, it is both desirable and medically required that the amount of x-ray radiation which penetrates the body be limited. However, too little exposure will result in an underexposure of the x-ray film, which will be useless as a medical diagnosis tool. One way to resolve this problem is to make repeated exposures of different view in an attempt to depict the subject adequately on x-ray film. This can, unfortunately, result in damage to the tissue as the result of excessive x-ray exposure. Industrial objects which contain electronic components may be damaged by long exposure to x-ray photons, and therefore require that low levels of radiation be used with relatively short exposures.
A variety of mechanisms and techniques have been developed in order to limit the amount of x-ray radiation that a subject may be exposed to in order to produce a usable image. One technique is to provide x-ray films having various contrast levels in order to achieve the desired image on film, even if a number of sheets of film are required to adequately depict the subject.
Another variation is the provision of scintillating screens which are provided with a variety of inorganic crystal phosphor elements embedded in the screen, and which is used to expose the x-ray film, or to illuminate a vidicon. The screens are operable to absorb the x-rays and to convert the x-rays to photons or visible light. A number of attempts have been made to render screens more sensitive to x-rays, such as providing screens in various thicknesses, providing screens which have various amounts of the phosphorous materials embedded therein and providing screens which have higher resolutions. All of these modifications require that, in order to produce a more intense image on the film or vidicon, the scintillating screens must be bombarded by large amounts of x-ray radiation which travel through the subject.
In all known scintillating screens, the x-rays which impinge on the screen contact the phosphor in or on the screen and produce visible light, which in turn causes light photons, along with the non-absorbed x-ray photons, to refract internally in the screen among the phosphorous particles, which causes a loss of optical imaging, resulting in a loss of resolution. Not all of the generated light exits the screen, as some of the light is dispersed while it travels laterally within the screen.
In the case of relatively thick screens, more x-ray photons are stopped and absorbed by the screen, which tends to create more visible light. However, thick screens are more likely to defuse light internally, resulting in lower resolution. Relative thin screens are less likely to defuse the light, but allow more of the x-ray photons to transit the screen unabsorbed. Thin screens therefore result in a higher resolution image, with less intensity, or brilliance, which requires a longer exposure. In some known systems, thin and thick screens are combined, with one screen on either side of the x-ray film, in order to produce an image which has a relatively short exposure time and results in an adequate image. However, the final resolution on the film ultimately requires sophisticated interpretation in order to discern the state of the subject which is depicted.
Another problem with scintillating screens which do not absorb the x-ray photons is transmission of the photons through the scintillating screen and beyond. While such problem is not particularly significant in the case of x-ray film, x-ray photons can have a detrimental effect on electronics, as may be present in camera systems.
An object of the invention is to provide an x-ray imaging system which has a very high resolution, absorbs nearly all of the x-ray photons impinging thereon, and produces a brilliant, visible-light image.
Another object of the invention is to provide an x-ray imaging system which is suitable for use with a camera system, to provide real-time x-ray images.
Another object of the invention is to provide an x-ray imaging system which utilizes a variable field of focus lens and charge-coupled device camera.